Computer Science
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.3205h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 15, pp.3205-3220
Computer Science
11
Scientific paper
We have used three different laser microprobe methods to explore the distribution of radiogenic 40 Ar in 1.0-1.5-mm biotite crystals from the ca. 1680 Ma Horse Mountain monzogranite of central Arizona. Incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near-plateau ages of 1190 Ma, showing no indication of intracrystalline 40 Ar inhomogeneity. In contrast, total fusion of twenty-five biotite fragments ( 100 m) yielded apparent ages ranging from 1006.7 to 1212.0 Ma. Detailed age mapping in the {001} plane of two crystals, with the laser focused to a minimum spot size, confirms that the age dispersion in the fragment data reflects the existence of 200 m.y. age gradients in single crystals. The two mapped crystals display very different age distribution patterns that suggest radiogenic 40 Ar loss through two mechanisms: volume diffusion on a scale comparable to that of the grain radius, and more rapid diffusion along discrete zones of high crystal defect density. Simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of 0.5 K/m.y. The Horse Mountain results, as well as previously published studies, indicate that conventional, incremental heating of hydrous phases can homogenize intracrystalline gradients in 40 Ar, thereby masking important details of the thermal history of analyzed samples. In contrast, detailed isotopic mapping studies offer a wealth of information, and will become more powerful with continued improvement in the spatial resolution of laser microprobes. Total-fusion studies of crystal fragment populations can be readily automated, making them less labor-intensive than mapping studies. Our preliminary experiment on a limited Horse Mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research.
Bowring Samuel A.
Hodges Kip V.
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